The UMTS system consists of a number of logical network elements that each have a defined functionality. In the standards, network elements are defined at the logical level, but this quite often results in a similar physical implementation, especially since there are a number of open interfaces (for an interface to be “open”, the requirement is that it has been defined to such a detailed level that the equipment at the endpoints can be from two different manufacturers). The network elements can be grouped based on similar functionality, or based on which sub-network they belong to. Functionally, the network elements are grouped into the Radio Access Network (RAN) which handles all radio-related functionality, and the core network (CN) which is responsible for switching and routing calls and data connections to external networks. To complete the system, a terminal device or user equipment (UE) provides an interface to a user.
From a specification and standardization point of view, both UE and UTRAN consist of completely new protocols, the design of which is based on the needs of the new Wideband Code Division Multiple Access (WCDMA) radio technology. On the contrary, the definition of the core networks is adopted from GSM (Global System for Mobile Communications). This gives the system with new radio technology a global base of known and rugged core network technology which accelerates and facilitates its introduction, and enables such competitive advantages as global roaming.
According to recent suggestions in standardization, an intra-domain connection of RAN nodes to several CN nodes may be used to connect CN nodes of multiple operators to a single RAN. A Non-Access Stratum (NAS) node selection function in the RAN nodes differentiates between CN nodes which may be of different operators. Preferably, the available network resource identifier (NRI) values are split between the operators. The NAS node selection function in the RAN node is configured to know which NM values belong to which operator. A mobile terminal which is not yet assigned to a CN node, there is no CN node configured for the NM indicated by the mobile terminal, is assigned to an available CN node of that operator which uses the indicated NM value. When no NRI value can be derived, the mobile terminal is as signed to a CN node selected from all available CN nodes. However, the selection of a CN node may result in the assignment to a CN node of a “wrong” operator as, for example, the NM may be derived from a TMSI which was allocated by a CN node of another network or by a CN node of an area which does not use the intra-domain connection of RAN nodes to multiple CN nodes. In such a case, the CN node rejects the attach or update request procedure. The NAS node selection function in the RAN node then selects an available CN node of another operator.
Currently, the TMSI or IMSI is provided in the attached message. Due to the above RAN sharing concept, each selected new CN node needs to derive the IMSI again. Deriving the IMSI includes radio signaling for identity request and authentication which implies significant delays. Such delays may exceed normal routing area update (RAU) procedure timings triggering the mobile terminal to resend its request message. Thus, network signaling is increased and a risk of misbehavior of the mobile terminal is introduced. Moreover, sending the IMSI several times over the radio interface is a security risk which should be avoided.